Periodic permanent magnet focusing system for electron discharge devices



Sept. 10, 1968 YOSHIHISA SUZUKI 3,

PERIODIC PERMANENT MAGNET FOCUSING SYSTEM FOR ELECTRON DISCHARGE DEVICES Filed May 18, 1966 2 Sheets-Sheet 1 PRIOR AR? f INVENTOR amman smml ATTORNEY Sept. 10, 1968 YOSHIHISA SUZUKI 3,401,295 PERIODIC PERMANENT MAGNET FOCUSING SYSTEM FOR ELECTRON DISCHARGE DEVICES 2 Sheets-Sheet 2 Filed May 18, 1966 PR/OR ART N\ SEE INVENTOR NS NS sNs ATTORNEY United States Patent 3,401,295 PERIODIC PERMANENT MAGNET FOCUS- Y ING SYSTEM FOR ELECTRON DISCHARGE DEVICES Yoshihisa Suzuki, Yokohama, Japan, assignor to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed May 18, 1966, Ser. No. 551,076

Claims priority, application Japan, May 21, 1965,

40/ 29,624 4 Claims. (Cl.'313-84) ABSTRACT OF THE DISCLOSURE Periodic permanent magnet focusing system for an electron discharge device formed of a plurality of unit permanent magnets of annular shape coaxially disposed in spaced relation with the direction of magnetization aligned in one direction, the unit permanent magnet being formed of pairs of adjacent half-sections with an annular magnetic pole piece interposed therebetween to reduce the length of the magnetic path in the space outside the assembly, and a method for manufacture thereof.

'- This invention relates to electron tubes with a beam focusing system of the type using periodic permanent magnets and more particularly to an electron tube of the kind described which has such a structure that the permanent magnets for the electron beam focusing can be magnetized after they have been mounted in place in the electron tube and the electron tube has been subsequently evacuated.

It is the primary object of the present invention to provide an electron tube with a beam focusing system of the type using periodic permanent magnets which is quite small in size andwhich has magnetic field distribution of good periodicity and of constant peak values.

. Other objects, advantages and features of the present invention will become apparent from the following description with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view showing the structure of prior art electron tubes with a beam focusing system of the type using conventional periodic permanent magnets;

FIG. 2 is a schematic sectional view showing the structure of a periodic permanent'magnet device forming the electron beam focusing system in a prior art electron tube of the kind described; I

FIG. 3 is a schematic sectional view showing the structure of a permanent magnet device forming the electron beam focusing system in one embodiment according to the invention; and

FIG. 4 is a schematic sectional view showing the structure of a permanent magnet device forming'the electron beam focusing system in another embodiment according to the invention.

Periodic permanent magnet devices are frequently fa vored for use in electron tubes such as traveling wave tubes and backward wave tubes for the focusing of an electron beam of high electron current density owing to the advantage that these devices are small in size and light in weight. A typical construction of the prior art electron tubes having such a device therein is shown in section in FIG. 1 by way of example. The electron tube shown in FIG. 1 includes an evacuated air-tight vessel 16 in which are accommodated a source of electrons 10, a collector 1 1 for collecting the beam of electrons "ice emitted from the electron source 10, an electron beam traveling passage 12, an electron beam focusing system 15 consisting of cylinder type permanent magnets 13 and magnetic pole pieces 14 disposed to surround the electron beam passage 12, an input waveguide 17, an output waveguide 18, and a slow wave circuit 19, shown here as a helix.

In the prior electron tube having the structure as shown in FIG. 1, its beam focusing system generally has such an arrangement that cylindrical permanent magnets of inverse polarity are alternately disposed in coaxial relation with each other. The magnetic electron beam focusing system in the prior electron tube has, for example, a periodic structure as shown in FIG. 2. This magnetic electron beam focusing system comprises a plurality of cylindrical permanent magnets 6 and a plurality of cylindrical permanent magnets '6 magnetized in a direction opposite to that of the permanent magnets 6. These permanent magnets 6 and 6 of inverse polarity are alternately and coaxially disposed along the central axis and a magnetic pole piece 7 having an electron beam passage '8 therein is interposed between each pair of the permanent magnets 6 and 6 of inverse polarity so that a periodic magnetic field can be established about the central axis. Since the permanent magnets which have already been magnetized in opposite directions to each other are assembled to form the above-described known type of magnetic electron beam focusing system, it is impossible to magnetize these masses of permanent magnet material in opposite directions to each other after mounting the non-magnetized masses of permanent magnet material in place. Therefore the masses of permanent magnet material must be mounted in place after having been magnetized, but degradation of magnetic properties of the permanent magnets is unavoidable due to the fact that the magnets are exposed to high temperature during the evacuation step. This is quite objectionable especially in the case of electron tubes for millimeter waves in which a strong magnetic field is required for an extremely high electron current density.

In an effort to solve the above problem, the inventors have previously proposed a periodic permanent magnet device in which masses of permanent magnet material can be magnetized after being mounted in place and after the step of subsequent evacuation. This periodic permanent magnet device had such a structure that non-magnetized cylindrical masses of permanent magnet material are mounted in a suitably spaced relation from each other and are magnetized in the same direction after the step of evacuation. However, the periodic permanent magnet device of the kind described has had some drawbacks in that the diameter of magnetic shielding plates at opposite ends thereof must-be made extremely larger than that of the cylinder type permanent magnets in order to obtain a periodic magnetic field of good periodicity and of constant peak value over the extremely large length. This kind of periodic permanent magnet device has thus been defective in that the size and dimensions of electron tubes become quite large. 7

With a view to eliminate the above defect of the prior periodic permanent magnet devices, the present invention provides a novel and improved periodic permanent magnet device for incorporation in an electron tube of the kind specified.

Now consider the distribution of magnetic fiux in case a plurality of hollow cylindrical magnets are all magnetized in the same direction and are disposed in coaxially spaced relation to each other. In this case the flux at the inner side of each hollow cylindrical magnet passes through magnetic pole pieces on opposite ends of the magnet and through its magnetic material to form a closed loop. On the other hand, the flux appearing across the gap between the hollow cylindrical magnets (this flux having a direction opposite to the direction of the flux appearing at the inner side of the hollow cylindrical magnet) passes at least through the material of the hollow cylindrical magnets on opposite sides of the gap and through theouter space of the hollow cylindrical magnets to form a closed loop. The longer the path length of the magnetic'flux through the outer space of the hollow cylindrical magnets, the periodicity or symmetry of the periodic magnetic field becomes poorer. The present invention therefore contemplates to shorten or restrict the above-described path length of the magnetic flux through the outer space in order to obtain a periodic magnetic field of constant peak value. More precisely, according to the present invention, hollow cylindrical magnets are each split into halves at the middle portion thereof and an annular magnetic pole piece having an outside diameter larger than the outside diameter of the above hollow cylindrical magnet is interposed between the halfsplit magnet sections so as to restrict the path length of the magnetic flux through the outer space of the hollow cylindrical magnets. The magnetic field at the inner side of the periodic permanent magnet device is not appreciably affected by this interposition of magnetic pole pieces and such influence becomes almost negligible by arranging the magnets and pole pieces in such a manner that the inside diameter of the magnetic pole piece is slightly greater than the inside diameter of the hollow cylindrical magnet.

The present invention will now be described in more detail with reference to FIGS. 3 and 4.

FIG. 3 shows the sectional structure and magnetic field distribution for a magnetic electron beam focusing means in one embodiment of the electron tube according to the invention. The magnetic electron beam focusing means comprises spaced pairs of hollow cylindrical permanent magnets 1 and 2 which are magnetized in the same direction, each forming a unit permanent magnet. On opposite ends of each pair of hollow cylindrical permanent magnets 1 and 2, there are disposed annular magnetic pole pieces 3 and 4, respectively, and an annular magnetic pole piece 5 is interposed between the hollow cylindrical magnets 1 and 2 in order to restrict the path length of magnetic flux through the outer space of the hollow cylindrical magnets. In the above structure, the magnetic flux inside the hollow cylindrical magnets passes through the same passage as in the previously described case, but the magnetic flux appearing across the gap between the pairs of hollow cylindrical magnets passes through the hollow cylindrical magnets on opposite sides of the gap, then through the magnetic pole piece 5 and through the outer space of the hollow cylindrical magnets to form a closed loop. It will be understood that the provision of the magnetic pole pieces 5 is so effective for greatly restricting the passage of magnetic flux through the outer space that the periodicity or symmetry of the periodic magnetic field would not be impaired at all, even if the permanent magnet device is so constructed as to have a great length.

FIG. 4 shows the sectional structure of a magnetic electron beam focusing means employed in another embodiment according to the invention, in which like ref erence numerals appearing in FIG. 3 are used to denote like parts. The arrangement shown in FIG. 4 differs from that shown in FIG. 3 in that a hollow cylindrical magnet 1' having no magnetic pole piece 5 for the restriction of the path length of magnetic flux through the outer space of the hollow cylindrical magnets is disposed between annular magnetic pole pieces 3 and 4 and between pairs of hollow cylindrical magnets 1 and 2. It will thus be ap preciated that the magnetic pole piece 5 need not be interposed in every pair of hollow cylindrical magnets and may be disposed in a suitably spaced relation.

It is likely that, as the magnetic field established about the central axis of the permanent magnet device becomes stronger, the density of magnetic flux passing through the magnetic pole pieces may be successively increased and as a result the permeability may gradually be lowered. In such a case, the effect of the provision of the magnetic pole pieces 3 and 4 for the intensification of the peak value of the periodic magnetic field is decreased with the result that the peak value of the magnetic field will be lowered to an extent that it is equal to the operating point of the magnet per se. According to the invention, this difiiculty can be remedied and a strong magnetic field can be established about the central axis by making the dimension 1 of the gap spacing larger than the dimension of the cylindrical magnet portion as shown in FIG. 4. In other words, by making the dimension I of the gap spacing larger than the dimension of the cylindrical magnet portion, the operating point of the magnet can be raised because of the fact that a reduced amount of magnetic flux will be supplied from the cylindrical magnet portion.

The above description has referred to a periodic permanent magnet device employing hollow cylindrical magnets but it will be understood that any other means may be employed in lieu thereof. For example, means such as bar magnets or means for creating a magnetic potential difference across the magnetic pole pieces 3 and 4 on opposite ends of the magnet may be employed and the magnetic pole piece 5 may be disposed at such a position at which it will have a middle magnetic potential.

From the foregoing description it will be understood that the inventive arrangement in which a magnetic pole piece 5 is interposed between the halved sections of a hollow cylindrical permanent magnet is quite effective in restricting the path length of magnetic flux through the outer space of the hollow cylindrical permanent magnet for thereby providing a periodic magnetic field of good periodicity and of constant peak value. According to the invention, the size of the electron tube can be made quite small because the good periodic magnetic field is used for beam focusing, even if it is quite long, and because there is no need of providing magnetic shielding plates of large diameter on opposite ends of periodic permanent magnet devices such as are employed in prior electron tubes of this type.

What is claimed is:

1. In an electron discharge device having electron beam generating means for emitting an electron beam along a prescribed path, a collector for collecting the electron beam emitted from said electron source and a periodic magnetic focusing system provided between said source and said collector surrounding at least a portion of the path of said electron beam progressing therebetween for focusing said electron beam, said focusing system comprising: a permanent magnet assembly formed of a plurality of unit permanent magnets of hollow cylindrical shape coaxially disposed in spaced relation with their direction of magnetization aligned in one direction, at least one of said unit permanent magnets being formed of a pair of adjacent half-sections confronting each other in a plane perpendicular to the beam path; and at least one annular magnetic pole piece coaxially interposed between said half-sections of said one unit permanent magnet and having an outside diameter larger than that of said one unit permanent magnet whereby reducing the length of magnetic path formed through the outer space of said permanent magnet assembly.

2. The combination according to claim 1, in which alternate one of the unit permanent magnets are formed, respectively, of a pair of adjacent half-sections, and a plurality of said annular magnetic pole pieces each coaxially interposed between the half-sections of a respective alternate one of said unit permanent magnets.

3. The combination according to claim 1, in which all of the unit permanent magnets are formed, respectively, of a pair of adjacent half-sections, and a plurality of said annular magnetic pole pieces each coaxially interposed between the half-sections of a respective unit permanent magnet.

4. An electron tube according to claim 1, in which the dimension of spacing between adjacent unit permanent magnets is made larger than the dimension l of said unit permanent magnet.

References Cited UNITED STATES PATENTS 3,013,172 12/1961 Ito 315-35 3,142,008 7/1964 Kajihara 31384 X 3,334,264 8/1967 Niclas 315-35 DAVID J. GAVIN, Primary Examiner.

V. LAFRANCHIE, Assistant Examiner. 

